There are many systems in which it is necessary to switch a signal generator from one frequency to another at high speed, with a minimum of time spent in transition between frequencies. Such a system is a frequency-hopping radio communication system in which, for purposes of communications security and jam-resistant operation, the carrier signal frequency is switched among as many as one hundred RF frequencies at a switching rate, for example, of one KHz. Since, in this example, there is a carrier frequency change every millisecond, it is important that as much of each millisecond be available for data transmission as is possible, by minimizing the unusable switching time between frequencies.
Most systems of this type, including the frequency-hopping systems mentioned above, use frequency synthesizers which include phase lock loops. Although these synthesizers are capable of high precision performance, their transition speed between frequencies, i.e., time to lock on a new frequency, can be intolerably slow and, in cases where the loop includes built-in frequency range restrictions such as mixers and filters in the loop feedback path, the loop may be entirely unable to lock on a new frequency.
In one prior art apparatus for rapidly changing frequency in a phase lock loop frequency synthesizer, disclosed in U.S. Pat. No. 4,105,948, issued Aug. 8, 1978, to Wolkstein, a frequency change is effected by opening the loop between the filter and the voltage-controlled oscillator (VCO) and applying an external tuning voltage to the VCO until the loop settles at the new frequency. This procedure is time consuming since the loop will not settle until the loop filter capacitor has charged or discharged to the new voltage, and this charge/discharge rate is limited by the low current drive capability of the phase detector.
It is also known generally to supply current directly to the loop filter capacitor of a phase lock loop to hasten the development of the control signal applied to the loop VCO. This current charges (or discharges) the capacitor to thereby provide a voltage approximating the tuning voltage required for the new frequency. Prior art phase lock loops with this tuning capability have employed the charge pump generally included within the phase detector circuitry. This charge pump is usually a very low current device, typically providing a maximum current of 30 milliamperes, and the prior art systems have boosted its current flow capability by either switching in a lower value current-limiting resistor in the output circuit, or by increasing the gain of the charge pump drive transistors by switching resistors in their base circuits. These switching modifications do not provide actual prepositioning-they merely hasten the time required for lock. True prepositioning would supply a known voltage across the filter capacitor, where that voltage is related to the new frequency sought to be locked on.